Solvents for dissolving an electrolyte salt for use in an electric double layer capacitor in which at least one of a positive electrode and a negative electrode is a polarizable electrode need to be stable at a withstand voltage of 3 V or more. From this point of view, a combined use of ethylene carbonate and propylene carbonate, which is a cyclic carbonate having a high oxidation potential (withstand voltage), has been proposed (Patent Document 1). The withstand voltage is limited to about 2.7 V, however.
Further, Patent Document 2 proposes using a non-aqueous solvent including sulfolane or a derivative thereof and a specific chain carbonic acid ester (chain carbonate) for the purpose of improvement in withstand voltage.    Patent Document 1: JP 2000-208372 A    Patent Document 2: JP 08-306591 A